Filaments are used to produce light and electrons. For example, in an x-ray tube, an alternating current can heat a wire filament formed in a coiled cylindrical or helical loop. Due to the high temperature of the filament, and due to a large bias voltage between the filament and an anode, electrons are emitted from the filament and accelerated towards the anode. These electrons form an electron beam. The location where the electron beam impinges on the anode is called the “electron spot.” It can be desirable that this spot be circular with a very small diameter. It can be desirable that this spot be in the same location on the anode in every x-ray tube that is manufactured.
The shape and placement of the filament in the x-ray tube affects the shape of the spot. Some filaments are very small, especially in portable x-ray tubes. Placing such small filaments, in precisely the same location, in every x-ray tube, can be a significant manufacturing challenge. Lack of precision of filament placement during manufacturing can result in an electron spot that is in different locations on the anode in different x-ray tubes. Placement of the filament also affects spot size and shape. Lack of precision of filament placement also results in non-circular spots and spots that are larger than desirable.
Shown in FIGS. 13-14 is a coiled cylindrical or helical wire filament 130. As this filament 130 heats and cools, the filament 130 can bend and change its shape, as shown in FIG. 14. As the filament changes shape, the electron spot can change both location and size. This can result in variability of x-ray tube performance over time. It is important that the shape and material of the filament allow for long filament life without filament deformation. Also, the coiled cylindrical or helical shape of the filament can result in non-circular electron spots.
In addition, a filament wire, with a consistent wire diameter, can be hottest at the mid-point 131 along the length of the wire. If there is a consistent wire diameter, the voltage drop or power loss is consistent along the wire, resulting in the same heat generation rate along the wire. The connections at the ends of the wire 132, however, essentially form a heat sink, allowing more heat dissipation, and cooler temperatures, at the each end of the wire. The mid-point of the wire 131 loses less heat by conduction than the wire ends and can be the hottest location on the filament wire. This high heat at the mid-point 131 can result in more rapid deterioration at the wire mid-point 131. As this mid-point 131 deteriorates, its diameter decreases, resulting in a larger power loss, higher temperatures, and an even greater rate of deterioration at this location. Due to the higher temperatures and more rapid wire deterioration at the mid-point 131 of the filament wire, most failures occur at this location. Such failures result in decreased tube life and decreased x-ray tube reliability.